Hi,

this my first build of a 4.2-rcN Linux-kernel and I see this...

[   24.001043] [drm] Memory usable by graphics device = 2048M
[   24.001118] [drm] Replacing VGA console driver
[   24.011642] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[   24.011646] [drm] Driver supports precise vblank timestamp query.
[   24.012480] vgaarb: device changed decodes:
PCI:0000:00:02.0,olddecodes=io+mem,decodes=io+mem:owns=io+mem
[   24.028005]
[   24.028014] ======================================================
[   24.028020] [ INFO: possible circular locking dependency detected ]
[   24.028027] 4.2.0-rc4-1-iniza-small #1 Not tainted
[   24.028032] -------------------------------------------------------
[   24.028038] modprobe/740 is trying to acquire lock:
[   24.028043]  (init_mutex){+.+.+.}, at: [<ffffffffa006af2d>]
acpi_video_get_backlight_type+0x17/0x164 [video]
[   24.028060]
[   24.028060] but task is already holding lock:
[   24.028068]  (&(&backlight_notifier)->rwsem){++++..}, at:
[<ffffffff810a6499>] __blocking_notifier_call_chain+0x39/0x70
[   24.028083]
[   24.028083] which lock already depends on the new lock.
[   24.028083]
[   24.028095]
[   24.028095] the existing dependency chain (in reverse order) is:
[   24.028103]
[   24.028103] -> #1 (&(&backlight_notifier)->rwsem){++++..}:
[   24.028113]        [<ffffffff810da56f>] lock_acquire+0xcf/0x280
[   24.028121]        [<ffffffff817d6269>] down_write+0x49/0x80
[   24.028129]        [<ffffffff810a6611>]
blocking_notifier_chain_register+0x21/0xb0
[   24.028138]        [<ffffffff814619f8>] backlight_register_notifier+0x18/0x20
[   24.028147]        [<ffffffffa006b010>]
acpi_video_get_backlight_type+0xfa/0x164 [video]
[   24.028158]        [<ffffffffa00a20e9>] 0xffffffffa00a20e9
[   24.028164]        [<ffffffff810020f8>] do_one_initcall+0x88/0x1c0
[   24.028172]        [<ffffffff817cc9d2>] do_init_module+0x61/0x1ec
[   24.028179]        [<ffffffff8111fb78>] load_module+0x2008/0x25c0
[   24.028187]        [<ffffffff81120256>] SyS_init_module+0x126/0x140
[   24.028194]        [<ffffffff817d8e32>] entry_SYSCALL_64_fastpath+0x16/0x7a
[   24.028202]
[   24.028202] -> #0 (init_mutex){+.+.+.}:
[   24.028211]        [<ffffffff810d989d>] __lock_acquire+0x1f5d/0x21c0
[   24.028218]        [<ffffffff810da56f>] lock_acquire+0xcf/0x280
[   24.028225]        [<ffffffff817d4275>] mutex_lock_nested+0x65/0x3c0
[   24.028233]        [<ffffffffa006af2d>]
acpi_video_get_backlight_type+0x17/0x164 [video]
[   24.028243]        [<ffffffffa006b0b2>]
acpi_video_backlight_notify+0x19/0x2f [video]
[   24.028253]        [<ffffffff810a629d>] notifier_call_chain+0x5d/0x80
[   24.028260]        [<ffffffff810a64b1>]
__blocking_notifier_call_chain+0x51/0x70
[   24.028269]        [<ffffffff810a64e6>]
blocking_notifier_call_chain+0x16/0x20
[   24.028278]        [<ffffffff81461d37>] backlight_device_register+0x197/0x240
[   24.028286]        [<ffffffffa062bb23>]
intel_backlight_register+0xb3/0x180 [i915]
[   24.028336]        [<ffffffffa05fe136>]
intel_modeset_gem_init+0x176/0x190 [i915]
[   24.028371]        [<ffffffffa0633112>] i915_driver_load+0xf12/0x14d0 [i915]
[   24.028406]        [<ffffffffa00ec861>] drm_dev_register+0xb1/0x100 [drm]
[   24.028425]        [<ffffffffa00ef62d>] drm_get_pci_dev+0x8d/0x1e0 [drm]
[   24.028439]        [<ffffffffa057e1fb>] i915_pci_probe+0x3b/0x60 [i915]
[   24.028463]        [<ffffffff814317e5>] local_pci_probe+0x45/0xa0
[   24.028470]        [<ffffffff81432bad>] pci_device_probe+0xcd/0x120
[   24.028477]        [<ffffffff8152d3a6>] driver_probe_device+0x136/0x2e0
[   24.028485]        [<ffffffff8152d5e4>] __driver_attach+0x94/0xa0
[   24.028491]        [<ffffffff8152b336>] bus_for_each_dev+0x66/0xa0
[   24.028499]        [<ffffffff8152cede>] driver_attach+0x1e/0x20
[   24.028506]        [<ffffffff8152cad8>] bus_add_driver+0x1b8/0x230
[   24.028513]        [<ffffffff8152e580>] driver_register+0x60/0xe0
[   24.028520]        [<ffffffff814310a4>] __pci_register_driver+0x64/0x70
[   24.028527]        [<ffffffffa00ef860>] drm_pci_init+0xe0/0x110 [drm]
[   24.028540]        [<ffffffffa069a0a0>] i915_init+0xa0/0xa8 [i915]
[   24.028563]        [<ffffffff810020f8>] do_one_initcall+0x88/0x1c0
[   24.028570]        [<ffffffff817cc9d2>] do_init_module+0x61/0x1ec
[   24.028577]        [<ffffffff8111fb78>] load_module+0x2008/0x25c0
[   24.028584]        [<ffffffff81120256>] SyS_init_module+0x126/0x140
[   24.028591]        [<ffffffff817d8e32>] entry_SYSCALL_64_fastpath+0x16/0x7a
[   24.028598]
[   24.028598] other info that might help us debug this:
[   24.028598]
[   24.028609]  Possible unsafe locking scenario:
[   24.028609]
[   24.028617]        CPU0                    CPU1
[   24.028622]        ----                    ----
[   24.028627]   lock(&(&backlight_notifier)->rwsem);
[   24.028633]                                lock(init_mutex);
[   24.028639]
lock(&(&backlight_notifier)->rwsem);
[   24.028648]   lock(init_mutex);
[   24.028654]
[   24.028654]  *** DEADLOCK ***
[   24.028654]
[   24.028664] 4 locks held by modprobe/740:
[   24.028669]  #0:  (&dev->mutex){......}, at: [<ffffffff8152d59b>]
__driver_attach+0x4b/0xa0
[   24.028681]  #1:  (&dev->mutex){......}, at: [<ffffffff8152d5a9>]
__driver_attach+0x59/0xa0
[   24.028693]  #2:  (drm_global_mutex){+.+.+.}, at:
[<ffffffffa00ec7d6>] drm_dev_register+0x26/0x100 [drm]
[   24.028711]  #3:  (&(&backlight_notifier)->rwsem){++++..}, at:
[<ffffffff810a6499>] __blocking_notifier_call_chain+0x39/0x70
[   24.028725]
[   24.028725] stack backtrace:
[   24.028734] CPU: 1 PID: 740 Comm: modprobe Not tainted
4.2.0-rc4-1-iniza-small #1
[   24.028742] Hardware name: SAMSUNG ELECTRONICS CO., LTD.
530U3BI/530U4BI/530U4BH/530U3BI/530U4BI/530U4BH, BIOS 13XK 03/28/2013
[   24.028753]  ffffffff8269fc40 ffff8801188134b8 ffffffff817cea9b
0000000000000011
[   24.028763]  ffffffff8269fc40 ffff880118813508 ffffffff817cbfc4
ffff8800d3a00b40
[   24.028774]  ffff880118813578 ffff8800d3a01410 0000000000000004
ffff8800d3a01410
[   24.028784] Call Trace:
[   24.028791]  [<ffffffff817cea9b>] dump_stack+0x4c/0x65
[   24.028797]  [<ffffffff817cbfc4>] print_circular_bug+0x20b/0x21c
[   24.028804]  [<ffffffff810d989d>] __lock_acquire+0x1f5d/0x21c0
[   24.028812]  [<ffffffff810da56f>] lock_acquire+0xcf/0x280
[   24.028819]  [<ffffffffa006af2d>] ?
acpi_video_get_backlight_type+0x17/0x164 [video]
[   24.028830]  [<ffffffff817d4275>] mutex_lock_nested+0x65/0x3c0
[   24.028837]  [<ffffffffa006af2d>] ?
acpi_video_get_backlight_type+0x17/0x164 [video]
[   24.028847]  [<ffffffffa006af2d>]
acpi_video_get_backlight_type+0x17/0x164 [video]
[   24.028857]  [<ffffffffa006b0b2>]
acpi_video_backlight_notify+0x19/0x2f [video]
[   24.028866]  [<ffffffff810a629d>] notifier_call_chain+0x5d/0x80
[   24.028873]  [<ffffffff810a64b1>] __blocking_notifier_call_chain+0x51/0x70
[   24.028880]  [<ffffffff810a64e6>] blocking_notifier_call_chain+0x16/0x20
[   24.028887]  [<ffffffff81461d37>] backlight_device_register+0x197/0x240
[   24.028920]  [<ffffffffa062bb23>] intel_backlight_register+0xb3/0x180 [i915]
[   24.028952]  [<ffffffffa05fe136>] intel_modeset_gem_init+0x176/0x190 [i915]
[   24.028984]  [<ffffffffa0633112>] i915_driver_load+0xf12/0x14d0 [i915]
[   24.028991]  [<ffffffff810d73e9>] ? mark_held_locks+0x69/0x90
[   24.028997]  [<ffffffff817d5cd0>] ? __mutex_unlock_slowpath+0xc0/0x180
[   24.029004]  [<ffffffff810d73e9>] ? mark_held_locks+0x69/0x90
[   24.029011]  [<ffffffff817d84d6>] ? _raw_spin_unlock_irqrestore+0x36/0x60
[   24.029018]  [<ffffffff810d7561>] ? trace_hardirqs_on_caller+0x151/0x1e0
[   24.029025]  [<ffffffff810d75fd>] ? trace_hardirqs_on+0xd/0x10
[   24.029039]  [<ffffffffa00ec861>] drm_dev_register+0xb1/0x100 [drm]
[   24.029052]  [<ffffffffa00ef62d>] drm_get_pci_dev+0x8d/0x1e0 [drm]
[   24.029076]  [<ffffffffa057e1fb>] i915_pci_probe+0x3b/0x60 [i915]
[   24.029082]  [<ffffffff814317e5>] local_pci_probe+0x45/0xa0
[   24.029089]  [<ffffffff81432a70>] ? pci_match_device+0xe0/0x110
[   24.029095]  [<ffffffff81432bad>] pci_device_probe+0xcd/0x120
[   24.029102]  [<ffffffff8152d3a6>] driver_probe_device+0x136/0x2e0
[   24.029109]  [<ffffffff8152d5e4>] __driver_attach+0x94/0xa0
[   24.029115]  [<ffffffff8152d550>] ? driver_probe_device+0x2e0/0x2e0
[   24.029124]  [<ffffffff8152b336>] bus_for_each_dev+0x66/0xa0
[   24.029134]  [<ffffffff8152cede>] driver_attach+0x1e/0x20
[   24.029144]  [<ffffffff8152cad8>] bus_add_driver+0x1b8/0x230
[   24.029153]  [<ffffffff8152e580>] driver_register+0x60/0xe0
[   24.029163]  [<ffffffff814310a4>] __pci_register_driver+0x64/0x70
[   24.029180]  [<ffffffffa00ef860>] drm_pci_init+0xe0/0x110 [drm]
[   24.029190]  [<ffffffffa069a000>] ? 0xffffffffa069a000
[   24.029218]  [<ffffffffa069a0a0>] i915_init+0xa0/0xa8 [i915]
[   24.029228]  [<ffffffff810020f8>] do_one_initcall+0x88/0x1c0
[   24.029238]  [<ffffffff817cc999>] ? do_init_module+0x28/0x1ec
[   24.029248]  [<ffffffff817cc9d2>] do_init_module+0x61/0x1ec
[   24.029257]  [<ffffffff8111fb78>] load_module+0x2008/0x25c0
[   24.029266]  [<ffffffff8111b6c0>] ? m_show+0x1a0/0x1a0
[   24.029278]  [<ffffffff817d9a73>] ? retint_kernel+0x10/0x10
[   24.029290]  [<ffffffff81120256>] SyS_init_module+0x126/0x140
[   24.029297]  [<ffffffff817d8e32>] entry_SYSCALL_64_fastpath+0x16/0x7a
[   24.030969] ACPI: Video Device [GFX0] (multi-head: yes  rom: no  post: no)
[   24.031157] input: Video Bus as
/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/LNXVIDEO:00/input/input11
[   24.031353] [drm] Initialized i915 1.6.0 20150522 for 0000:00:02.0 on minor 0
[   24.041224] fbcon: inteldrmfb (fb0) is primary device

Not sure which area is the root cause, so I added acpi and drm/i915 folks.

dmesg and kernel-config are attached.
More infos?
Please let me know what you need.

Regards,
- sed@ -